Actually, looking at the following, if his salary is £12k a year, he may not have to pay any tax on his £4K interest. A simple call to the tax office should clear it up.
Personal Allowance
You can use your
Personal Allowance to earn interest tax-free if you have not used it up on your wages, pension or other income.
Starting rate for savings
You may also get up to £5,000 of interest and not have to pay tax on it. This is your starting rate for savings.
The more you earn from other income (for example your wages or pension), the less your starting rate for savings will be.
If your other income is £17,570 or more
You’re not eligible for the starting rate for savings if your other income is £17,570 or more.
If your other income is less than £17,570
Your starting rate for savings is a maximum of £5,000. Every £1 of other income above your Personal Allowance reduces your starting rate for savings by £1.